Secondary cells using a rechargeable hydrogen storage negative electrode are known in the art. These cells operate in a different manner than lead-acid, nickel-cadmium or other prior art battery systems. The hydrogen storage electrochemical cell utilizes a negative electrode that is capable of reversibly electrochemically storing hydrogen. In one exemplification the cell employs a positive electrode of nickel hydroxide material, although other positive electrode materials may be used. The negative and positive electrodes are spaced apart in an alkaline electrolyte, and may include a suitable separator, spacer, or membrane therebetween.
Upon application of an electrical current to the negative electrode, the negative electrode material (M) is charged by the absorption of hydrogen: EQU M+H.sub.2 O+e.sup.- M--H+OH.sup.- (Charge)
Upon discharge, the stored hydrogen is released to provide an electric current: EQU M--H+OH.sup.- M+H.sub.2 O+e.sup.- (Discharge)
The reactions are reversible.
The reactions that take place at the positive electrode are also reversible. For example, the reactions at a conventional nickel hydroxide positive electrode as utilized in a hydrogen rechargeable secondary cell or battery are: EQU Ni(OH).sub.2 +OH.sup.- NiOOH+H.sub.2 O+e.sup.- (Charge), and EQU NiOOH+H.sub.2 O+e.sup.- Ni(OH).sub.2 +OH.sup.- (Discharge).
A cell utilizing a electrochemically rechargeable hydrogen storage negative electrode offers important advantages over conventional secondary batteries. Rechargeable hydrogen storage negative electrodes offer significantly higher specific charge capacities (ampere hours per unit mass and ampere hours per unit volume) than do either lead negative electrodes or cadmium negative electrodes. As a result of the higher specific charge capacities, a higher energy density (in watt hours per unit mass or watt hours per unit volume) is possible with hydrogen storage batteries than with the prior art conventional systems, making hydrogen storage cells particularly suitable for many commercial applications.
Suitable active materials for the negative electrode are disclosed in U.S. Pat. No. 4,551,400 to Sapru, Hong, Fetcenko and Venkatesan for HYDROGEN STORAGE MATERIALS AND METHODS OF SIZING AND PREPARING THE SAME FOR ELECTROCHEMICAL APPLICATION incorporated herein by reference. The materials described therein store hydrogen by reversibly forming hydrides. The materials of the '400 patent have compositions of: EQU (TiV.sub.2-x Ni.sub.x).sub.1-y M.sub.y
where x is between 0.2 and 1.0, y is between 0.0 and 0.2 and M=Al or Zr; EQU Ti.sub.2-x Zr.sub.x V.sub.4-y Ni.sub.y
where x is between 0.0 and 1.5, and y is between 0.6 and 3.5; and EQU Ti.sub.1-x Cr.sub.x V.sub.2-y Ni.sub.y
where x is between 0.0 and 0.75, and y is between 0.2 and 1.0.
Reference may be made to U.S. Pat. No. 4,551,400 for further descriptions of these materials and for methods of making same.
Other suitable materials for the negative electrode are disclosed in commonly assigned U.S. Pat. No. 4,728,586 issued Mar. 1, 1988 in the names of Srinivasen Venkatesan, Benjamin Reichman, and Michael A. Fetcenko for ENHANCED CHARGE RETENTION ELECTROCHEMICAL HYDROGEN STORAGE ALLOYS AND AN ENHANCED CHARGE RETENTION ELECTROCHEMICAL CELL; and U.S. Pat. No. 4,623,597, to Sapru, et al for RECHARGEABLE BATTERY AND ELECTRODE USED THEREIN, both of which are incorporated herein by reference. As described in the '586 patent, one class of particularly desirable hydrogen storage alloys comprises titanium, vanadium, nickel, and at least one metal chosen from the group consisting of aluminum, zirconium, and chromium. The preferred alloys described in the '586 patent are alloys of titanium, vanadium, nickel, zirconium, and chromium, especially alloys having the composition represented by the formula: EQU (Ti.sub.2-x Zr.sub.x V.sub.4-y Ni.sub.y).sub.1-z Cr.sub.z
where x is between 0.0 and 1.5, y is between 0.6 and 3.5, and z is an effective amount less than 0.20.
The hydrogen storage alloy material may be formed by a number of different techniques such as from a high temperature melt, melt spinning or other metallurgical process. While a high temperature melt is preferred, different melting techniques may be employed with varying degrees of success. For example, early studies on hydrogen storage alloys were done using non-consumable arc melting apparatus. Arc melting techniques provide several advantages over other melt processes. These advantages include: (1) great versatility in terms of the types of materials which can be processed; (2) limited reactivity during melting; and (3) relatively low initial equipment costs for small scale arc melt systems. Unfortunately, as the size of the system required increases, equipment costs grow exponentially, until such a system becomes prohibitively expensive. Consequently, the economical size limitation of these types of apparatus is approximately 30 grams. Thus, this type of system is ideally suited for laboratory testing of sample and experimental materials; indeed most literature discussing hydrogen storage alloy materials makes reference to non-consumable arc melting apparatus for purposes of fabricating said materials.
While non-consumable arc melting possess the above described advantages in laboratory use, practically speaking it is almost impossible to scale-up for large scale production processes. This being the case, most researchers working in the metal hydride field have contemplated using consumable arc melting for scale-up production of metal hydride, hydrogen storage alloys. Consumable arc melting typically involves making a press powder compact of the raw materials into a rod shaped configuration. This compact rod is then consumed and melted by passing a high current arc into the end of the rod. Thus, an approximately ten foot long compacted rod having a diameter of approximately three inches is slowly passed through a chamber wherein the end of the rod is melted by an arc discharge and the melted material then drips, from the end of the rod, wherein melt is caught in a cooling vessel, solidifying into the final alloy ingot.
While consumable arc melting could be employed for use with hydrogen storage alloys such as those referred to hereinabove, it has several inherent disadvantages. Chief among these disadvantages is the inherent hazard present in the high current arc which is typically employed in such a manner so as to pass from the consumable cathode to a water cooled copper lined anode. This high current arc has been documented to have melted through the water cooled copper lining, thereby contacting the water and resulting in a rather violent reaction. While this is not a normal occurrence, it has been documented and has contributed to reduced acceptance of this type of technique. Other disadvantages associated with consumable arc melting include: (1) homogeneity, i.e., while this technique has been employed to alloy materials, it is typically not employed with alloys where a single component does not constitute at least 90 percent of the overall material. The hydrogen storage alloy materials discussed hereinabove are typically alloyed much more extensively. Indeed, the majority component in some of the metal hydride, hydrogen storage alloy materials discussed hereinabove can make up as little as 33 percent of the overall composition. Thus, it is likely that the final alloy prepared by consumable arc melting would have component gradients in composition, thus preventing commercial use; (2) preparation of the powder compact, i.e., in preparing the consumed rod employed in the consumable arc melting process, it is particularly important to make sure that uniformity in distribution of the raw materials be precisely controlled in order to achieve an compositionally homogenous final alloy. This is of course particularly important in situations wherein, as mentioned hereinabove, a very high degree of alloying is required. While precise processes for preparing the powder compact are well known, it is also well known that in order to prepare an adequate compact for a highly alloy material, it is impossible to use the most inexpensive forms of the raw materials to be alloyed, i.e., inexpensive (and plentiful) forms such as turnings and other irregularly shaped materials; and (3) process efficiency on the whole with this type of process tends to be very costly. A great deal of power extended to melt the raw materials is directed towards heating the water cooling medium rather than heating the raw materials. Additionally, the process has relatively low throughput and is fairly labor intensive. Further, the process is highly operator sensitive and therefore susceptible to the production of high quantities of scrap material, thereby significantly increasing the overall cost of the final alloy products.
The disadvantages inherent in the melt techniques discussed hereinabove are substantially overcome by the vacuum induction melting technique detailed hereinbelow. While this technique provides several advantages over prior art techniques, it also posed technical challenges to economical fabrication of hydrogen storage alloys. The most significant challenge posed was that of providing a crucible means in which to carry out the melt/alloy of the raw materials. Invariably, induction methods have failed because of the rapid, often violent reaction of one or more of the reactive component metals with the container or crucible used for the melt. Different types of crucible means have been suggested in conjunction with induction melting techniques. For example, U.S. Pat. No. 4,079,523 to Sandrock for "Iron, Titanium, Mishmetal Alloys for Hydrogen Storage" discusses a method for the preparation of an iron titanium mishmetal alloy which is used for hydrogen storage. Generally speaking, the Sandrock alloy is prepared by air melting an iron charge in a clay graphite crucible, thereafter a charge of titanium is added to the molten iron along with a deoxidizing mishmetal. While the Sandrock method may be successful for fabricating iron-titanium hydrogen storage alloys, the introduction of oxygen as by air into the metal hydride, hydrogen storage materials disclosed in, for example, Sapru, et al produces materials having inferior hydrogen storage capacity. Further, the clay-graphite crucible described in Sandrock cannot be employed in conjunction with the hydrogen storage materials disclosed hereinabove, which materials react with clay-graphite making containment difficult, (if not impossible), and preventing the crucible from being reused. Further, impurities are introduced into the final alloy.
The teaching of U.S. Pat. No. 2,548,897 to Kroll, et al is limited to the disclosure of a process for melting group IVa transition metals such as titanium, zirconium and hafnium in a graphite crucible. While this disclosure possesses some teaching which is relevant to the instant disclosure, it is important to note that the materials taught by, for example, Sapru, et al generally contain less than 30% combined titanium and zirconium. Therefore the teaching of Kroll, et al cannot be expanded to teach the invention disclosed herein. Further, Kroll, et al acknowledge the presence of carbon and carbides in the ingot of material which results from the melt process. It is noteworthy that in the metal hydride, hydrogen storage alloy materials discussed hereinabove, carbon and carbides therein are considered contaminants. These contaminants deleteriously effect the hydrogen storage capacity of the materials, and the performance parameters of said materials in electrochemical cells; therefore these contaminants are unacceptable for inclusion in metal hydride, hydrogen storage alloys, and must be minimized.
U.S. Pat. No. 3,529,958 to Buehler discloses a method of forming a titanium-nickel based alloy in a graphite crucible. While this reference has some teaching that may be of value to the method disclosed herein, it is to be noted that the Buehler reference requires that prior to the actual melt process, a pre-alloy process be conducted in order to coat the wall of the graphite crucible to prevent interaction thereof with the titanium-nickel alloy therewith. This pre-alloy process requires that a titanium-nickel starter plate be disposed in the bottom of the melting crucible in order to first melt, thereby preventing direct contact between the component metals and the crucible walls. The method disclosed herein does not require the use of a pre-alloy in order to prevent interaction of the component materials with the crucible. Further, the "TiNi base-type alloys" disclosed in the Buehler reference are specifically directed towards TiNi based alloys which further include Co and/or Fe.
Accordingly, it can be seen that there exists a need for an economical, safe method for the alloy fabrication of a highly alloyed, metal hydride, hydrogen storage alloy material.